Electromagnet with a field-responsive control system

ABSTRACT

In an electromagnet, to generate a force which is independent from the armature position, an on-off control system for the excitation current is provided which operates as a function of the magnetic field intensity. The control system causes the excitation current to oscillate between two values and thus have a constant mean value according to a preset desired value. The timelag of the inductivity or its change between two close values is utilized for measuring the magnetic flux density and for a comparison with a desired value.

United States Patent Dick [45] June 20, 1972 s41 ELECTROMAGNET WITH AFIELD- 3,165,675 1/1965 Shapiro ..317/010. 6 RESPONSIVE CONTROL SYSTEM3,170,095 2/1965 Goldstein... 17/010. 6

3, 41, l [72] Inventor: Heinrich Dick, Heidenheim, Germany 2 002 3/ 966Smith 317mm 6 [73] Assignee: Voith Getrieb KG, Heidenheim (Brenz), rimay E.\aminer-L. T. Hix

Germany AttorneyEdwin E. Greigg [22] Filed: April 22, 1971 ABSTRACT I. 67 [211 App N0 13 in an electromagnet, to generate a force which isindependent from the armature position, an onoff control system for the[30] Foreign Application Priority Data excitation current is providedwhich operates as a function of April 22 1970 Germany 20 19 345] themagnetic field intensity. The control system causes the excitationcurrent to oscillate between two values and thus have 521 u.s.c1..317/123, 317/148.5 R, 317/010. 6 a constant mean value according to aP desired value- [51 1 Int. Cl. ..H0lh 47/32 The timelag of theinductivity or its Change between two close [58] Fi ld of Se h 317/123,DIG, 6 values is utilized for measuring the magnetic flux density andfor a comparison with a desired value. 56 R ierences Cited 1 e 7 Claims,5 Drawing Figures UNITED STATES PATENTS 1,817,431 8/1931 Anderson..3l7/D IG. 6

I Z I68 2 I65 5\ 7- 8 14a mun-30mm rm 3,671,814

SHEET 10F 2 INVENTOR.

SHEET 2 [BF 2 ELECTROMAGNET WITH A FIELD-RESPONSIVE CONTROL SYSTEMBACKGROUND OF THE INVENTION This invention relates to an electromagnetwith a stationary, ironclad coil and a movable armature projectingthrough an open location of the iron cladding; said armature is drawn tothe iron cladding by the magnetic field generated by virtue of currentflowing through the coil. Assuming a constant excitation current, uponmovement of the armature towards the iron cladding, the flux densityincreases. The electromagnet is further of the type that includes aforce accumulator (gravitational force, spring, pressure cushion) urgingthe armature to move away from the iron cladding.

It is known to generate a distance-independent linear force by means ofa plunger coil device which is characterized by a circular cylindricalmagnetic field generated by a permanent magnet or by a direct currentand having radially extending short magnetic field lines into which athin-layer coil is axially immersed. Depending on the magnitude of thecurrent flowing through the plunger coil, the latter is exposed to agreater or lesser axially orientated force which is independent form theposition of the coil provided that all turns of the plunger coil aredisposed in the undisturbed magnetic field. A plunger coil device ofthis kind, however, is capable of generating only comparatively smallforces. Plunger coil devices designated for larger forces areunproportionately large and heavy. The best plunger coil devices areable to produce a force corresponding to approximately 0.4 times theirown dead weight. It is also a disadvantage that the required controlpower is very high and that the coil constitutes the moving part.Apartfrom their large weight, plunger coil devices are very expensivedue to their complex structure and the requirements for high precisionin the manufacture of the coil.

Although relatively large forces may be generated by a small magnet ofthe kind mentioned heretofore, the attracting force on the armaturedepends to a large extent on its position. Thus, the attracting forceincreases hyperbolically as the armature approaches the coil core.Although, by a suitable design of the magnetic field (partial field lineshunt) an approximately linear force/distance curve may be obtained inzones and thus the effect of distance within each zone is substantiallyeliminated, such design restricts the magnet to the zone of minimumpower. To permit the generation of larger forces independently of thedistance would necessitate the provision of a magnet of very largedimensions. The disadvantageous results are extensive spacerequirements, large weight and a high current consumption. Moreover, astrokeindependent attracting force can be achieved only along shortdistances of displacement caused by the attracting force.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention toprovide an improved electromagnetic device of simple and light-weightstructure which is adapted to generate a relatively large,distance-independent linear force and with which the magnitude and thedirection of said force may be altered in a rapid manner.

Briefly stated, according to the invention, there is provided anelectromagnetic device of the aforeoutlined type which includes a meansfor regulating the excitation current. Said means comprises a transducerelement which is responsive to the magnetic field intensity and which isdisposed in the air gapbetween the armature and the iron cladding. Thetransducer element, which may be a Hall-generator or a field resistor,upon command by a desired value setter, regulates the excitation currentto obtain a magnetic field excitation which is constant at least as faras its average value with respect to time is concerned thus resulting ina constant,'distance-independent magnetic force.

The invention will be better understood as well as further objects andadvantages of the invention will become more apparent from the ensuingdetailed specification of several ex- BRIEF DESCRIPTION OF THE DRAWINGFIG. 1 is a circuit diagram of an embodiment of the invention, includingan electromagnet in longitudinal section;

FIG. 2 is a circuit diagram of a further embodiment of the invention,including, in longitudinal section, an electromagnet designed as asolenoid valve;

FIG. 3 is a circuit diagram of still another embodiment of theinvention, including an associated electromagnet in longitudinalsection;

FIG. 4 is a circuit diagram of still a further embodiment of theinvention, including an associated electromagnet in longitudinal sectionand g FIG. 5 is a circuit diagram of still another embodiment.

DESCRIPTION OF THE EMBODIMENTS Turning now to FIG. 1, there is shown anelectromagnet generally indicated at 1, having a coil 2, and ironcladding, 3, 3' surrounding the coil 2 and an armature 4'axially movabletherein. The radial face 4a of armature 4, together with a projection 5integral with the iron cladding 3 in the coil core defines an air gap 6.A spring 7 is disposed between the projection 5 and the radial face 4aof the armature 4 to urge the latter outwardly thus tending to increasethe air gap 6.

A field resistor 8, responsive to the magnetic field strength, isaffixed (e.g..glued to the end face of the projection 5. The fieldresistor 8 may be constituted by a semiconductor element which altersits resistance in the same sense as the change of a traversing magneticflux. Thus, the voltage drop across the field resistor is a directmeasure of the attracting force of the armature. The two terminals ofthe resistor 8 are broughtout through a bore provided in cladding 3.

The electronic circuit associated with the magnet 1 comprises aregulator part9 and a switch part 10, which are connected throughconductors 12 and 13 to a voltage source such 1 as a battery 11. Theregulator part incorporates a resistance bridge circuit formed of thefield resistor 8, as well as a fixed resistor 14 and a variable resistor15, 16. Between the resistors 14 and 8 there is disposeda measuringpoint 148, whereas another measuringpoint 165 is located between the tworesistor parts 16 and 15 of the variable resistor l5, 16. From thebattery 11 a constant voltage is applied to a feed point 168 between theresistors 16 and 8 and to a feed point 145 between resistors 14 and 15.The two potentials, of which that at 165 maybe arbitrarily set, arecompared with each other in the resistancebridge circuit. This tappedvoltage is applied through a series resistor 23 to one input E of anamplifier V which has two inputs H5 and E and an output A. The measuringpoint 148 is connected through the series resistor 24 to the input i E.When the resistance of the field resistor is reduced assuming initialpotentials at both measuring points 148 and 165 the potential at point148 will increase relative to that at point 165. As a result, thepotential increasesat output A relative to the terminal 10 of theamplifier, assuming an initial state of identical potentials. Theamplifier output A is connected to the base of a transistor T Theincrease of potential at the amplifier output A and thus at the base ofthe transistor T caused by a drop of the resistance of the fieldresistor 8, allows current to flow through the collector-emitter leg ofthe transistor T thus driving a power transistor 1,. As a result, thecollector-emitter leg of the power transistor T connected in series withthe coil 2 of the magnet 1 between the feed conductors 12 and 13 isrendered conductive and thus the coil feed circuit is closed.

The aforedescribed response to a change in the magnetic field strengthin air gap 6 takes place practically without a time lag. Stated indifferent terms, the coil 2 is energized as Starting from the precedinglow magnetic flux, a stronger I magnetic field will thus beprogressively built up in the air gap 6 so that, among other effects,the armature '4 is slightly disemplary embodiments taken in conjunctionwith the drawing. placed towards the projection 5 against the forceexerted by the spring 7. The increase of the magnetic field intensitycauses an increase of the resistance of the field resistor 8. Suchincrease, in turn results in a dropping of the potential at themeasuring point 148 relative to the potential at the measuring point165. The dropping of the potential continues as long as the potential at148 is smaller than the potential at 165. A negative input voltage willthus be applied to the amplifier V and accordingly, the potential at theoutput A will become increasingly negative relative to the zero point i0. As the output potential passes the i point value in the direction ofnegative values at the base of transistor T,, the collectoremitter legof the latter becomes non-conductive, whereby the power transistor T iscut off. This results in a de-energization of the coil 2.

By virtue of a bypass diode D connected parallel with the coil 2 in thedirection of the preceding current flow, the magnetic field in the airgap 6 decays exponentially and relatively slowly. The simultaneousincrease of the air gap due to the outward movement of the armature 4 asurged by spring 7, reduces the flux passing through the field resistor 8and thus, the resistance of the latter drops. This reduction inresistance once again causes the coil 2 to be energized. This results inan increase of the resistance of the field resistor 8 which, in turn,causes the coil 2 to be de-energized, etc.

The aforedescribed regulation of the magnetic field intensity may beregarded as a two-point control which oscillates with a systemicfrequency. This frequency comprises squarewave pulses of identicalamplitude and represents the on-off switching frequency for the coilcurrent. A magnetic field ex citation of greater or lesser intensitywill be needed dependent upon the position of the armature 4 and thepulling force to be exerted by the magnet. Accordingly, a lower orhigher frequency will be set by the system. This is governed by thevoltage drop which is detemiined by the resistance of the fieldintensity-responsive resistor 8 and which is compared with a set(desired) potential difference. By virtue of the latter, it is possiblein practice to compare and regulate the magnetic flux with anotherdesired value.

A practical application of the magnet according to the invention isillustrated in FIG. 2. The control circuit shown therein is fullyequivalent to that illustrated in FIG. 1. In the resistor bridge circuitof FIG. 2, instead of the variable resistor 15, 16 of FIG. 1, two fixedresistors a and 16a are provided between the two feed points 145 and168. The amplifier input E is connected to the variable output of afunction generator 14" which may be, for example, a sinusoidal generatoradjustable with respect to frequency and amplitude or a generatoradapted to supply from a given moment, upon receipt of a command signal,a defined ramp function with adjustable parameters. Or, a tachogenerator may be used which produces an rpm-analogous potentialdifference at the feed point 145 with respect to the other feed point168.

The magnet l of FIG. 2 is an electrohydraulic transducer wherein thearmature is formed of a piston 4' of a pressure limiting valve generallyindicated at 17. The displacement of the piston 4 in response to themagnetic field results in a greater or lesser restriction of thevolumetric flow delivered by the pump through the throttle formed by thecontrol lands 18 and 19. Depending on the attracting force of the magnet1 a greater or lesser pressure is built up upstream of the throttle(i.e. in the delivery side of the pump 20). The pressure which isindicated by the pressure gauge 21 may be directed through theconnecting conduit 22 to any desired loads and may be limited as to itsmaximum value by means of the pressure limiting resistor 24. Thegenerated pressure is also transmitted to the radial end face of piston4' in the air gap 6 through a radial and an axial bore provided in thepiston 4'. In this manner, in the air gap 6 a pressure cushion isgenerated which acts against the attracting force of the magnet. Themagnitude of said pressure cushion is immaterial, provided acounterforce is produced which will counteract the attracting force ofthe magnet. In order to ensure that the forces urging the piston 4outwardly, and thus the attracting forces generated by the magnet, donot become excessive and that the generated pressure does not affect theentire cross section of the piston, there is provided a reducing pin 7which is slidably disposed in the axial bore of piston 4' in a fluidtight manner and which,' exposed to the generated pressure, abuts theprojection 5. The pressures which may be controlled by theelectrohydraulic transducer 1', 17 are very large. Pressures of up to 50kg/cm or more may be controlled with ease by means of an electromagnethaving a weight of approximately 200 g. The oscillation superimposed onthe entire system, enables the transducer to respond very rapidly andpermits a corresponding output signal to follow with great rapidity thechanges in the input values.

Turning now to FIG. 3, in the control system for regulating the magneticforce, a so-called Hall generator 8 is used which, similarly to thefield resistor 8 of FIGS. 1 and 2, is also disposed in the air gap 6.The generator 8 requires a constant feed current which is supplied by avoltage source 25. At the two output terminals of the generator 8' thereappears a voltage which, assuming a constant feed current, isproportional to the magnetic flux traversing the generator. If themagnet coil fed directly by the power amplifier through a diode D isenergized, the Hall generator will supply a voltage which increases withthe inward movement of the armature and the corresponding increase offlux density. The amplifier inputs HE, E are connected to two circuitsin which current flows in opposite directions. One circuit, formed bythe lower resistor part 15' of a variable resistor 15', 16', a seriesresistor 23' and the amplifier input, is adjustable at will to set thedriving potential difference by varying the location of the tappingpoint The outer circuit is formed by the Hall generator 8' and a seriesresistor 24. The polarity of the Hall generator in the circuit must besuch that the Hall voltage opposes the driving potential differenceacross the resistor part 15'. When the Hall voltage exceeds thepotential difference across the resistor 15, the potential of the point168' shifts towards the negative range so that an input signal of apolarity in accordance with the terminal designation appears at theamplifier input E, +E. The input signal causes a corresponding amplifiedpotential increase with respect to $0 at the amplifier output A. Becauseof the blocking effect of the diode D,, the shift of the amplifieroutput into the positive range causes the magnet coil to bede-energized. In response to the now decreasing magnetic field and theoutward movement of the magnet armature 4 as urged by the spring 7, theHall voltage will drop. At one moment during this process the Hallvoltage will become smaller than the voltage increase across theresistor l5, and the point 168' will become positive relative to theother measuring point 148. An input signal with a polarity opposite tothat of the temtinal designation will then appear at the amplifier input--E, +E resulting in the appearance at the amplifier output A of acorrespondingly amplified powerful potential drop relative to fl) sothat the magnet coil 2 is energized through the diode D,. Theaforedescribed energization and de-energization is repetitive similarlyto the embodiment described in connection with FIG. 1. Here too, asystemic switching frequency will appear.

The desired value of coil excitation for the magnet according to FIG. 3(i.e. the force to be exerted by the magnet) may be adjusted on thevariable resistor 15', 16 or may be preset by a function generatorprovided instead of the variable resistor similarly to FIG. 2. Or, theauxiliary voltage source 25 may be replaced by a function generator ofthe kind heretofore described for setting the desired value for coilexcitation. The Hall voltage generated by the Hall generator isproportional to the product of its feed current and magnetic flux sothat the magnetic intensity can also be affected by the control currentwhich flows through the Hall generator.

By means of the embodiment illustrated in FIG. 4 a voltage responsive tothe magnetic field intensity is generated in a different manner. Themagnet system is provided with an auxiliary winding 2" disposed withinthe coil 2'. This auxiliary wind ing may be regarded as the secondarywinding of a transformer, the secondary voltage of which depends on thechange, with respect to time, of the field line density of thesurrounding magnetic field. As already described, during the control ofthe excitation current a systemic oscillation takes place. The excitercoil 2' is supplied practically only with the positive half waves of asquare-wave voltage whose mean value with respect to time is equal tothe excitation current required for the specified armature pull. Thismeans that the magnetic field is continuously increased and thendecreased through the bypass diode D. The said magnetic field isdetected by the auxiliary coil 2" on the terminals of which a voltageappears which is proportional to the change of magnetic flux withrespect to time. Since it is desired, however, to ob tain a voltagewhich is proportional to the flux itself, the voltage delivered by thecoil has to be integrated with respect to time. For this purpose thereis provided an amplifier V,, the inputs of which are connected with theoutput terminals of the auxiliary winding 2" and which is associatedwith a feedback capacitor C. The capacitive feedback of the amplifieroutput to one of the amplifier inputs gives the amplifier itsintegrating characteristics. Thus, between measuring points 148" and168" of the resistance bridge circuit a generator is provided whichdelivers a voltage proportional to the magnetic flux in the magnet 1".The effect of this generator and the mode of operation of thisembodiment is equivalent to that of the precedingly describedembodiment.

FIG. 5 shows a practical application of the invention wherein the magnetis void of any separate magnetic field-sensitive transducer. The role ofthe transducer necessary for the regulation of the excitation current istaken over by the magnet coil itself which is shown as an inductance Land as an ohmic resistance R L is the momentary inductance of the magnetsystem depending on the position of the armature of the magnet and thecoil size, while R is the ohmic resistance of the copper windings. Thecircuit system is based on the principle that the excitation current inthe magnet system can be measured as a voltage drop across a measuringresistor R which is serially connected to the coil L, R,,. This currentor the measuring voltage taken from the terminals of the measuringresistor R contains a constant direct voltage component resulting fromthe voltage drop across the two ohmic resistances R and R in addition toa voltage component which is proportional to the product of theinduction and the change of the excitation current and which varies inaccordance with the buildup and decay of the magnetic field. Theaforenoted constant direct voltage component of the measuring signalinitially obtained is first suppressed by means of a differentiatingcircuit formed of a capacitor 19 and a resistor 30 and is thenintegrated in a first integrating stage V,, C The output volt age of thelatter is proportional to the change of flux in the magnet system L, RThis signal is again integrated in a second integrating stage V C toprovide a voltage which is proportional to the flux density prevailingin the magnet. Since the circuit is based on a voltage which isproportional to the product of the momentary induction and the momentaryexcitation current, it follows that the signal obtained from the outputof the second integrating stage is, too, dependent upon the position ofthe armature. Thus, similarly to the previously described embodiments,the armature position too is taken into consideration in themeasurement. The signal finally obtained is compared with a desiredpotential adjustable at a variable resistor 31, and, depending onwhether the signal or the desired potential predominates, the coil L, Ris connected to or disconnected from the current supply through theswitching amplifier V and the two transistors T and T Although thecircuit according to FIG. 5 is more complex from a technological pointof view, it is advantageous in that the inventive principle can bepracticed with conventional electromagnets without any modification ofthe magnet system itself.

In the different embodiments described hereinabove, the momentarymagnetic field strength in the magnet is measured in four different waysand the work coil is energized or deenergized depending on whether thesignal characterizing the magnetic field strength is greater or smallerthan an adjustable desired value. Since the buildup of a magnetic fieldor the diminishing of an existing magnetic field are phenomena whichhave a timely course and since the magnetic field excitation, withpreset and closely adjacent values, requires a certain period of time,this period, as proposed by the invention, may be utilized for signalmeasurement and for comparison with desired values. Depending on theresults of the comparison, corrective measures are taken. The inertiainherent in the inductance provides sufficient time for a twopointregulation which is a feature utilized in accordance with the invention.

It is thus seen that the transistor circuit connected to the output ofthe amplifier in the embodiments is, in fact, a solidstate on-off powerswitch which converts the regulating system for the excitation currentinto a two-point control system. This feature involves two advantages.in the first place, the on-off control limits the tum-on period of thepower transistor in the power dissipation range to the required minimum(due to the steep voltage increase at the transistor the range of powerdissipation is very rapidly traversed), so that the losses at the powertransistor are maintainedat a very small value. In the second place, themagnet armature is subjected to small oscillations which eliminatestatic friction and hysteresis effects.

What is claimed is:

1. In an electromagnet of the type that includes (a) a magnet coil, (b)an iron cladding surrounding said magnet coil, (c) an armature movablewithin said coil and defining an air gap with a part of said cladding,(d) means for supplying an excitation current to said coil to generate amagnetic field passing through said air gap and exerting an inwardlydirected attracting force to said armature and (e) means exerting anoutwardly directed force on said armature; the intensity of saidmagnetic field being dependent upon the position of said armature fromsaid cladding, the improvement comprising a circuit means for regulatingsaid excitation current; and circuit means including A. a magnetic fieldintensity-responsive means disposed within said cladding and respondingat least indirectly to the intensity of said magnetic field,

B. setting means for obtaining signals corresponding to a desired valueof magnetic force,

C. comparator means for comparing the output signals of said magneticfield intensity-responsive means with those of said setting means and AD. switching means for regulating the admission of said excitationcurrent to said magnet coil in response to the output signals of saidcomparator means for providing in said air gap a magnetic field being ofconstant magnitude at least as to a mean value with respect to time andbeing independent from the position of said armature.

2. An improvement as defined in claim 1, wherein said means defined in(A) responds directly to the intensity of said magnetic field and isdisposed in said air gap.

3. An improvement as defined in claim 2, including A. a field resistorconstituting said magnetic field intensityresponsive means,

B. means for applying the voltage drop across said field resistor tosaid comparator means for comparing said voltage drop with a desiredpotential difference prevailing at said setting means and C. anamplifier having input means for receiving the output signals of saidcomparator means, said amplifier having output means connected to saidswitching means for supplying the latter with an amplifier outputcurrent which is at least decreased when said voltage drop exceeds saiddesired potential difference and which is increased when said desiredpotential difference exceeds said voltage drop.

4. An improvement as defined in claim 3, including A. a resistancebridge circuit containing 1. said field resistor,

2. a variable resistor constituting said setting means,

B. means for connecting two diagonal measuring points of said resistancebridge circuit to two inputs of said amplifier,

C. a first transistor having a base to which the output signals of saidamplifier are applied; said first transistor having a collector-emitterleg,

D. a second, or power transistor having a base to which the signals ofthe collector-emitter leg of said first transistor are applied; saidsecond transistor having a collectoremitter leg; said first and secondtransistors forming part of said switching means and E. a direct voltagesource connected to diagonal feed points of said resistance bridgecircuit and, through the collector-emitter leg of said secondtransistor, to said magnet coil.

5. An improvement as defined in claim 1, wherein said means defined in(A) responds indirectly to the intensity of said magnetic field.

6. An improvement as defined in claim 5, including A. an auxiliarywinding disposed inside said magnet coil and constituting said magneticfield intensity-responsive means; said auxiliary winding generatesoutput signals induced therein by the excitation current flowing in saidmagnet coil and Y B. an integrating circuit connected to said auxiliarywinding; said integrating circuit is connected to said comparator meansfor applying thereto its output signals.

7. An improvement as defined in claim 5, wherein said magnetic fieldintensity-responsive means is constituted by said magnet coil itself;and improvement further includes A. a differentiating circuit connectedto said magnet coil to receive therefrom output signals that include avoltage component due to the self-induction in response to theexcitation current; said differentiating circuit is adapted to suppressa direct voltage component of the coil output signal due to the ohmicresistance of said coil,

B. a first integrating circuit connected to said differentiating circuitfor delivering a voltage proportional to the change of the magnetic fluxin said air gap and C. a second integrating circuit connected to saidfirst integrating circuit for delivering a voltage proportional to theproduct of the momentary value of the excitation current and themomentary value of the inductance of said magnet coil; said last namedvoltage is applied to said comparator means including said settingmeans.

1. In an electromagnet of the type that includes (a) a magnet coil, (b)an iron cladding surrounding said magnet coil, (c) an armature movablewithin said coil and defining an air gap with a part of said cladding,(d) means for supplying an excitation current to said coil to generate amagnetic field passing through said air gap and exerting an inwardlydirected attracting force to said armature and (e) means exerting anoutwardly directed force on said armature; the intensity of saidmagnetic field being dependent upon the position of said armature fromsaid cladding, the improvement comprising a circuit means for regulatingsaid excitation current; and circuit means including A. a magnetic fieldintensity-responsive means disposed within said cladding and respondingat least indirectly to the intensity of said magnetic field, B. settingmeans for obtaining signals corresponding to a desired value of magneticforce, C. comparator means for comparing the output signals of saidmagnetic field intensity-responsive means with those of said settingmeans and D. switching means for regulating the admission of saidexcitation current to said magnet coil in response to the output signalsof said comparator means for providing in said air gap a magnetic fieldbeing of constant magnitude at least as to a mean value with respect totime and being independent from the position of said armature.
 2. Animprovement as defined in claim 1, wherein said means defined in (A)responds directly to the intensity of said magnetic field and isdisposed in said air gap.
 2. a variable resistor constituting saidsetting means, B. means for connecting two diagonal measuring points ofsaid resistance bridge circuit to two inputs of said amplifier, C. afirst transistor having a base to which the output signals of saidamplifier are applied; said first transistor having a collector-emitterleg, D. a second, or power transistor having a base to which the signalsof the collector-emitter leg of said first transistor are applied; saidsecond transistor having a collector-emitter leg; said first and secondtransistors forming part of said switching means and E. a direct voltagesource connected to diagonal feed points of said resistance bridgecircuit and, through the collector-emitter leg of said secondtransistor, to said magnet coil.
 3. An improvement as defined in claim2, including A. a field resistor constituting said magnetic fieldintensity-responsive means, B. means for applying the voltage dropacross said field resistor to said comparator means for comparing saidvoltage drop with a desired potential difference prevailing at saidsetting means and C. an amplifier having input means for receiving theoutput signals of said comparator means, said amplifier having outputmeans connected to said switching means for supplying the latter with anamplifier output current which is at least decreased when said voltagedrop exceeds said desired potential difference and which is increasedwhen said desired potential difference exceeds said voltage drop.
 4. Animprovement as defined in claim 3, including A. a resistance bridgecircuit containing
 5. An improvement as defined in claim 1, wheRein saidmeans defined in (A) responds indirectly to the intensity of saidmagnetic field.
 6. An improvement as defined in claim 5, including A. anauxiliary winding disposed inside said magnet coil and constituting saidmagnetic field intensity-responsive means; said auxiliary windinggenerates output signals induced therein by the excitation currentflowing in said magnet coil and B. an integrating circuit connected tosaid auxiliary winding; said integrating circuit is connected to saidcomparator means for applying thereto its output signals.
 7. Animprovement as defined in claim 5, wherein said magnetic fieldintensity-responsive means is constituted by said magnet coil itself;and improvement further includes A. a differentiating circuit connectedto said magnet coil to receive therefrom output signals that include avoltage component due to the self-induction in response to theexcitation current; said differentiating circuit is adapted to suppressa direct voltage component of the coil output signal due to the ohmicresistance of said coil, B. a first integrating circuit connected tosaid differentiating circuit for delivering a voltage proportional tothe change of the magnetic flux in said air gap and C. a secondintegrating circuit connected to said first integrating circuit fordelivering a voltage proportional to the product of the momentary valueof the excitation current and the momentary value of the inductance ofsaid magnet coil; said last named voltage is applied to said comparatormeans including said setting means.